The present disclosure is related to marking and printing methods and systems, and more specifically to methods and systems for recovering a dampening solution (such as water-based fountain fluid) in a variable lithography marking or printing system.
Offset lithography is a common method of printing. (For the purposes hereof, the terms “printing” and “marking” are used interchangeably.) In a typical lithographic process the surface of a print image carrier, which may be a flat plate, cylinder, belt, etc., is formed to have “image regions” of hydrophobic and oleophilic material, and “non-image regions” of a hydrophilic material. The image regions correspond to the areas on the final print (i.e., the target substrate) that are occupied by a printing or marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by said marking material. The hydrophilic regions accept and are readily wetted by a water-based dampening fluid (commonly referred to as a fountain solution, and typically consisting of water and a small amount of alcohol as well as other additives and/or surfactants). The hydrophobic regions repel dampening solution and accept ink, whereas the dampening solution formed over the hydrophilic regions forms a fluid “release layer” for rejecting ink. Therefore the hydrophilic regions of the printing plate correspond to unprinted areas, or “non-image areas”, of the final print.
The ink may be transferred directly to a substrate, such as paper, or may be applied to an intermediate surface, such as an offset (or blanket) cylinder in an offset printing system. The offset cylinder is covered with a conformable coating or sleeve with a surface that can conform to the texture of the substrate, which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the imaging plate. Sufficient pressure is used to transfer the image from the offset cylinder to the substrate. Pinching the substrate between the offset cylinder and an impression cylinder provides this pressure.
The above-described lithographic and offset printing techniques utilize plates which are permanently patterned, and are therefore useful only when printing a large number of copies of the same image (long print runs), such as magazines, newspapers, and the like. However, they do not permit creating and printing a new pattern from one page to the next without removing and replacing the print cylinder and/or the imaging plate (i.e., the technique cannot accommodate true high speed variable data printing wherein the image changes from impression to impression, for example, as in the case of digital printing systems). Furthermore, the cost of the permanently patterned imaging plates or cylinders is amortized over the number of copies. The cost per printed copy is therefore higher for shorter print runs of the same image than for longer print runs of the same image, as opposed to prints from digital printing systems.
Lithography and the so-called waterless process provide very high quality printing, in part due to the quality and color gamut of the inks used. Furthermore, these inks—which typically have a very high color pigment content (typically in the range of 20-70% by weight)—are very low cost compared to toners and many other types of marking materials. However, while there is a desire to use the lithographic and offset inks for printing in order to take advantage of the high quality and low cost, there is also a desire to print variable data from page to page. Heretofore, there have been a number of hurdles to providing variable data printing using these inks. Furthermore, there is a desire to reduce the cost per copy for shorter print runs of the same image. Ideally, the desire is to incur the same low cost per copy of a long offset or lithographic print run (e.g., more than 100,000 copies), for medium print run (e.g., on the order of 10,000 copies), and short print runs (e.g., on the order of 1,000 copies), ultimately down to a print run length of 1 copy (i.e., true variable data printing).
One problem encountered is that offset inks have too high a viscosity (often well above 50,000 cps) to be useful in nozzle-based inkjet systems. In addition, because of their tacky nature, offset inks have very high surface adhesion forces relative to electrostatic forces and are therefore almost impossible to manipulate onto or off of a surface using electrostatics. (This is in contrast to dry or liquid toner particles used in xerographic/electrographic systems, which have low surface adhesion forces due to their particle shape and the use of tailored surface chemistry and special surface additives.)
Efforts have been made to create lithographic and offset printing systems for variable data in the past. One example is disclosed in U.S. Pat. No. 3,800,699, incorporated herein by reference, in which an intense energy source such as a laser to pattern-wise evaporate a dampening solution.
In another example disclosed in U.S. Pat. No. 7,191,705, incorporated herein by reference, a hydrophilic coating is applied to an imaging belt. A laser selectively heats and evaporates or decomposes regions of the hydrophilic coating. A water based dampening solution is then applied to these hydrophilic regions, rendering them oleophobic. Ink is then applied and selectively transfers onto the plate only in the areas not covered by dampening solution, creating an inked pattern that can be transferred to a substrate. Once transferred, the belt is cleaned, a new hydrophilic coating and dampening solution are deposited, and the patterning, inking, and printing steps are repeated, for example for printing the next batch of images.
In known systems, following transfer of the inked pattern to the substrate, the cleaning step completely removes the dampening solution and any remaining ink. Thorough and complete cleaning is required to prevent residual elements from prior images (“ghosting”) and other artifacts from affecting the image to be printed. Knife-edge cleaning (effectively, scraping) systems, wiper or brush systems, non-contact cleaning process such as high pressure rinsing or solvent cleaning, and other techniques are used to fully clean the print image carrier. However, stripping dampening solution and residual ink together from the print image carrier means that reuse of either dampening solution or ink is impracticable or most commonly not possible.
One possible approach to recovery of dampening solution would be to remove the dampening solution after forming the inked image on the print image carrier but before the transfer nip at which ink is transferred to the substrate. This presents an ink-only interface between the print image carrier and substrate, or a totally fluid-free nip for blank regions of an image. However, it is generally undesirable to expose the print image carrier surface to direct physical contact with the substrate. For example, in embodiments in which the substrate is paper, the abrasive surface of the paper can limit the working lifespan of the print image carrier. Systems and methods are needed to improve the recapture and reuse of the dampening solution without negatively affecting print image quality or image carrier lifespan.